Over the years, various means for monitoring and determining the condition of used lubricating oils have been developed. Such methods have been used by workers involved in the development of lubricants and additives therefor, by equipment manufacturers and by operators of equipment in the field. Such tests include acid number, wear metal content, color, change in viscosity with time and the like.
A test which has been used to determine the dispersive condition of a used oil, either a test oil or one which has been used in field service, is to assess the appearance of a spot of the oil which has been placed on an absorbent material. This method involves placing a measured drop of the oil on a sheet of absorbent material, usually a material such as chromatography paper, allowing the spot to develop, then examining the spot for the presence of undispersed sludge. The oil sample appears as a circular spot on the absorbent material. As the oil is absorbed it will form a particular and characteristic pattern. This pattern is then interpreted to determine the condition of the oil. One such test, which involves only a visual assessment of the oil spot, is described in C. A. Bailey and J. S. Aarons, "The Lubrication Engineers Manual", First Edition, United States Steel Corporation, pages 72-73 (1971).
Visual interpretation of the oil spots can provide a qualitative assessment of the condition of the oil sample. However, it has been found that the size of the components of the oil spot provide considerable information regarding the oil condition. In the past, workers generally have measured the approximate diameter of the various components of the oil spot using a ruler or other simple measuring device. There exists a need for a simpler, quantitative method for measuring the oil spots, assigning a pass or fail rating to same, and classifying the relative performance of various oils.